Is there a way to fix this, as it is broken. If I have a main account of bobby@mydomain.com and I set the default address to :fail: it should allow mail to bobby@mydomain.com and reject anything else that does not exist on the server.

IE:

Main Account: bobby
Other email: webmaster@mydomain.com
another one: suzzy@mydomain.com
another one: tim@mydomain.com

If I set Default Address to :fail: then....

Emails to webmaster, suzzy, tim, AND bobby should pass through since they exist, but mail to anything_else@mydomain.com should be bounced ( if using :fail: ) or eaten ( if using :blackhole )
This is the way I read the documentation for CPanel.